GA
⌘K
⌘K
Infrastructure · 1 min read

Writing Your First Linux Kernel Module

A practical guide to writing, building, and loading a Linux kernel module from hello world to character devices.

#linux-kernel#kernel-modules#device-drivers#systems-programming

Why Kernel Modules?

Kernel modules let you extend Linux without recompiling the entire kernel. Device drivers, filesystems, network protocols all loadable at runtime.

Hello Kernel

#include <linux/module.h>
#include <linux/kernel.h>

static int __init hello_init(void) {
    printk(KERN_INFO "Hello from kernel space\n");
    return 0;
}

static void __exit hello_exit(void) {
    printk(KERN_INFO "Goodbye from kernel space\n");
}

module_init(hello_init);
module_exit(hello_exit);
MODULE_LICENSE("GPL");

Build, Load, Verify

make -C /lib/modules/$(uname -r)/build M=$(pwd) modules
sudo insmod hello.ko
dmesg | tail -1    # "Hello from kernel space"
sudo rmmod hello

Key Concepts

  • No libc: You’re in kernel space. Use printk, kmalloc, copy_to_user
  • No floating point: Kernel doesn’t save FPU state
  • Concurrency: Multiple CPUs, interrupts, preemption use spinlocks, mutexes
  • One bug = kernel panic: Test in a VM first